1. Field of the Invention
The present invention relates to a fuel injector and to a diesel engine provided with the fuel injector.
2. Description of the Related Art
As an accumulator fuel injector, a fuel injector comprising one accumulator accumulating a pressurized fuel, fuel injection valves to which the fuel is supplied from the accumulator, and thereby the fuel injection valves are opened and the fuel is injected, fuel feeding pipes for feeding the fuel accumulated in the accumulator to the fuel injection valves, and one valve mechanism which is provided to the fuel feeding pipe and which opens and allows the flow of the fuel in the accumulator to the fuel injection valves.
In the fuel injector, regardless of increase and decrease of a load applied to the fuel injector, a fuel injection rate is substantially fixed during fuel injecting. Therefore, in this fuel injector, smoke emission during low load is reduced due to high injection rate. However, at the early period of a fuel injection during high load, since a large amount of fuel is injected, the amount of NOx contained in exhaust gas increases, and environmental problems may occur. Therefore, countermeasures, which can achieve the reduction in smoke emission during low load and decrease in NOx during high load, have been desired.
In addition, in the diesel engine comprising this fuel injector, when the valve mechanism is broken and the diesel engine does not operate, the diesel engine must be stopped and the valve mechanism must be changed. Therefore, for example, when the diesel engine is used for generating electric power, during the change operation, electric power is temporarily not supplied. In addition, when the diesel engine is a main engine of a ship, the ship is temporarily crippled.
Furthermore, for example, Unexamined Japanese Patent Application, First Publication No. Hei 11-182380 discloses an accumulator fuel injector which controls the fuel injection rate at an early period of a fuel injection. The accumulator fuel injector comprises fuel injection valves each of which comprises a solenoid valve for controlling fuel injection therein, a low pressure accumulator, a high pressure accumulator, and a solenoid valve for changing pressure which opens and allows the flow of fuel from the high pressure accumulator to the fuel injection valves.
In this fuel injector, when the solenoid valve for controlling fuel injection is opened, fuel, which is applied to a hydraulic piston closing a needle valve, is recovered in a fuel tank via a leak pipe. Thereby, the needle valve is pushed upwardly by the fuel in a fuel pool and fuel is injected from a nozzle hole to a combustion chamber. In contrast, when the solenoid valve for controlling fuel injection is closed, the hydraulic piston of the needle valve is pressed, the needle valve is closed. Specifically, at the early period of the fuel injection, the solenoid valve for changing pressure is closed and the solenoid valve for controlling fuel injection is opened, fuel is supplied from the low pressure accumulator to the fuel injection valves. At the later phase of the fuel injection, the solenoid valve for changing pressure and the solenoid valve for controlling fuel injection are both opened, and thereby fuel in the high pressure accumulator is supplied to the fuel injection valves.
In this fuel injector, in order to close the needle valve, it is necessary to close the solenoid valve for controlling fuel injection and to apply fuel pressure to the hydraulic piston. In particular, in medium and large diesel engines using heavy oil as fuel, since heavy oil does not have a fixed viscosity, a time lag from when fuel pressure is applied to the hydraulic piston and to when the needle valve is closed varies, depending on the viscosity of the heavy oil. Due to this, a fuel injection is not completed immediately and late combustion phase becomes worse. Thereby, a problem arises in that an exhaust gas temperature and fuel consumption increase.